.TH std::priority_queue 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::priority_queue \- std::priority_queue

.SH Synopsis
   Defined in header <queue>
   template<

       class T,
       class Container = std::vector<T>,
       class Compare = std::less<typename Container::value_type>

   > class priority_queue;

   The priority queue is a container adaptor that provides constant time lookup of the
   largest (by default) element, at the expense of logarithmic insertion and
   extraction.

   A user-provided Compare can be supplied to change the ordering, e.g. using
   std::greater<T> would cause the smallest element to appear as the top().

   Working with a priority_queue is similar to managing a heap in some random access
   container, with the benefit of not being able to accidentally invalidate the heap.

.SH Template parameters

   T         - The type of the stored elements. The program is ill-formed if T is not
               the same type as Container::value_type.
               The type of the underlying container to use to store the elements. The
               container must satisfy the requirements of SequenceContainer, and its
               iterators must satisfy the requirements of LegacyRandomAccessIterator.
               Additionally, it must provide the following functions with the usual
               semantics:
   Container -   * front(), e.g., std::vector::front(),
                 * push_back(), e.g., std::deque::push_back(),
                 * pop_back(), e.g., std::vector::pop_back().

               The standard containers std::vector (including std::vector<bool>) and
               std::deque satisfy these requirements.
               A Compare type providing a strict weak ordering.

               Note that the Compare parameter is defined such that it returns true if
   Compare   - its first argument comes before its second argument in a weak ordering.
               But because the priority queue outputs largest elements first, the
               elements that "come before" are actually output last. That is, the front
               of the queue contains the "last" element according to the weak ordering
               imposed by Compare.

.SH Member types

   Member type     Definition
   container_type  Container
   value_compare   Compare
   value_type      Container::value_type
   size_type       Container::size_type
   reference       Container::reference
   const_reference Container::const_reference

.SH Member objects

   Member name  Definition
   Container c  the underlying container
                \fI(protected member object)\fP
   Compare comp the comparison function object
                \fI(protected member object)\fP

.SH Member functions

   constructor   constructs the priority_queue
                 \fI(public member function)\fP
   destructor    destructs the priority_queue
                 \fI(public member function)\fP
   operator=     assigns values to the container adaptor
                 \fI(public member function)\fP
.SH Element access
   top           accesses the top element
                 \fI(public member function)\fP
.SH Capacity
   empty         checks whether the container adaptor is empty
                 \fI(public member function)\fP
   size          returns the number of elements
                 \fI(public member function)\fP
.SH Modifiers
   push          inserts element and sorts the underlying container
                 \fI(public member function)\fP
   push_range    inserts a range of elements and sorts the underlying container
   (C++23)       \fI(public member function)\fP
   emplace       constructs element in-place and sorts the underlying container
   \fI(C++11)\fP       \fI(public member function)\fP
   pop           removes the top element
                 \fI(public member function)\fP
   swap          swaps the contents
   \fI(C++11)\fP       \fI(public member function)\fP

.SH Non-member functions

   std::swap(std::priority_queue) specializes the std::swap algorithm
   \fI(C++11)\fP                        \fI(function template)\fP

.SH Helper classes

   std::uses_allocator<std::priority_queue> specializes the std::uses_allocator type
   \fI(C++11)\fP                                  trait
                                            \fI(class template specialization)\fP
   std::formatter<std::priority_queue>      formatting support for std::priority_queue
   (C++23)                                  \fI(class template specialization)\fP

     Deduction guides \fI(since C++17)\fP

.SH Notes

       Feature-test macro       Value    Std                   Feature
   __cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion
                                               for containers

.SH Example


// Run this code

 #include <functional>
 #include <iostream>
 #include <queue>
 #include <string_view>
 #include <vector>

 template<typename T>
 void pop_println(std::string_view rem, T& pq)
 {
     std::cout << rem << ": ";
     for (; !pq.empty(); pq.pop())
         std::cout << pq.top() << ' ';
     std::cout << '\\n';
 }

 template<typename T>
 void println(std::string_view rem, const T& v)
 {
     std::cout << rem << ": ";
     for (const auto& e : v)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

 int main()
 {
     const auto data = {1, 8, 5, 6, 3, 4, 0, 9, 7, 2};
     println("data", data);

     std::priority_queue<int> max_priority_queue;

     // Fill the priority queue.
     for (int n : data)
         max_priority_queue.push(n);

     pop_println("max_priority_queue", max_priority_queue);

     // std::greater<int> makes the max priority queue act as a min priority queue.
     std::priority_queue<int, std::vector<int>, std::greater<int>>
         min_priority_queue1(data.begin(), data.end());

     pop_println("min_priority_queue1", min_priority_queue1);

     // Second way to define a min priority queue.
     std::priority_queue min_priority_queue2(data.begin(), data.end(), std::greater<int>());

     pop_println("min_priority_queue2", min_priority_queue2);

     // Using a custom function object to compare elements.
     struct
     {
         bool operator()(const int l, const int r) const { return l > r; }
     } customLess;

     std::priority_queue custom_priority_queue(data.begin(), data.end(), customLess);

     pop_println("custom_priority_queue", custom_priority_queue);

     // Using lambda to compare elements.
     auto cmp = [](int left, int right) { return (left ^ 1) < (right ^ 1); };
     std::priority_queue<int, std::vector<int>, decltype(cmp)> lambda_priority_queue(cmp);

     for (int n : data)
         lambda_priority_queue.push(n);

     pop_println("lambda_priority_queue", lambda_priority_queue);
 }

.SH Output:

 data: 1 8 5 6 3 4 0 9 7 2
 max_priority_queue: 9 8 7 6 5 4 3 2 1 0
 min_priority_queue1: 0 1 2 3 4 5 6 7 8 9
 min_priority_queue2: 0 1 2 3 4 5 6 7 8 9
 custom_priority_queue: 0 1 2 3 4 5 6 7 8 9
 lambda_priority_queue: 8 9 6 7 4 5 2 3 0 1

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to     Behavior as published              Correct behavior
   LWG 307  C++98      Container could not be        allowed
                       std::vector<bool>
   LWG 2566 C++98      Missing the requirement for   ill-formed if T is not the same
                       Container::value_type         type as Container::value_type
                       priority_queue takes a
   LWG 2684 C++98      comparator                    added
                       but lacked member typedef for
                       it

.SH See also

   vector       dynamic contiguous array
                \fI(class template)\fP
   vector<bool> space-efficient dynamic bitset
                \fI(class template specialization)\fP
   deque        double-ended queue
                \fI(class template)\fP
